Considerable work has been performed on embedding electrical cabling and optical fibers into structural composite materials. Typically, this embedded cabling is integrated with sensor arrays in order to measure and report environmental and material conditions such as temperature, strain, and onset of damage. Another application for embedded cabling is for creating an integrated optical bus for high bandwidth data communication throughout a composite structure. Such an integrated bus can be used to transmit information between external or embedded sensors, actuators, processors, displays, communication devices, and other components associated with the composite structure. In such circumstances, the composite structure acts as a structural local area network (LAN).
One example application for a structural LAN is in a modern military ground vehicle, which has a series of complex, interdependent subsystems such as propulsion, communication, and weapons. Currently, conductive wiring is traced between components in the vehicle. This technique has several disadvantages. Cutouts in the structure are necessary to provide a pathway for wiring between onboard devices. Electrical cabling, especially for complex systems with many components, can introduce significant weight and volume. Wireless communication is one alternative to electrical wiring, but is susceptible to interference, jamming, and interception.
Replacing the external cabling in the military vehicle with integrated busses embedded in a composite structure offers a number of advantages over wired or wireless LANs, such as lower weight and volume, higher bandwidth, and insusceptibility to interference, jamming, and interception. However, an important challenge is the connectorization of the embedded cabling to external devices. Interfacing with embedded busses using conventional techniques requires physical connectorization. For electrical busses, coaxial or multi-pin interfaces are most common. For optical busses, ferrules and mechanically interlocking connectors are typically implemented. For example, to implement this approach for embedded optical fibers, a short length of fiber end is typically traced out of the structure. These “pigtails” are extremely fragile and bulky, and have greatly limited the practical application of embedded optical fibers.
One method for remote querying of embedded components in a military vehicle is via RF communications. However, in applications where interference, jamming, or interception of signals is possible, methods using RF communication are unfavorable. Another method used for remote querying of embedded components is to use a graded index lens (GRIN). This approach typically requires the interrogation light source to have line-of-sight access to the end face of the optical fiber, which limits interrogation to the edges of composite structures. For most practical structural network applications, however, edge access is very limited.
What would be desirable, but has heretofore not been implemented, are noninvasive, normal incidence, free-space data porting structural composites with integrated optical busses.